Numerous different film-type evaporator constructions are known and are characterized in that the product to be thickened or evaporated runs, in the form of a film-like layer, over the evaporation surface, which leads to high k-values (heat transfer coefficients). A uniform film thickness over the entire evaporation length can be achieved in that the rotor has ledges with wiper edges, which move past the inner wall at a limited distance therefrom. Film-type evaporators are particularly suitable for the concentration, thickening or distillation of heat-sensitive products, e.g. liquid or liquid/solid mixtures.
Such film-type evaporators operate in a single-pass process, that is, the starting product is only passed once over the evaporating surface and the concentrate produced is drawn off. High efficiency levels during thickening and/or better separating effects can be obtained only by adopting a multistage construction, in that either several evaporators are connected in series or several evaporating surfaces are arranged in step-like manner in a single container and the concentrate is transferred from one evaporating surface to the other. The multistage arrangement in the case of such film-type evaporators either leads to high space requirements or to considerable sizes, so that such evaporating plants are only suitable for a large-scale use.
In, for example, U.S. Pat. No. 2,927,634, a lower film-type evaporator in a lower part of a container thereof, is provided with a rotating rotor with radially extending blade-like ledges, which form the wiper or wiping edges. The container is conically constructed in the lower part and the wiper edges are correspondingly inclined and at a limited distance from the container inner wall. As a result the starting product introduced into the container at the upper end of the rotor and which runs down the inner container wall is uniformly distributed in thin film form. Concentrate is drawn off at the bottom of the lower part. Even with type of evaporator the thickening efficiency is limited as a result of the single pass. An increase in the thickening capacity can only be achieved by increasing the overall height.